Among devices that measure constituents of body fluids, those that measure constituents of blood are known. Such body fluid constituents measurement devices are used for purposes such as measurement of the blood sugar levels of patients with diabetes. Such body fluid constituents measurement devices are equipped with test paper which changes color when reacted with blood glucose, blood is applied to the test paper, and the change in color is optically measured by means of colorimetry for quantification of the blood glucose level.
In such blood glucose measurement devices, the blood glucose level is determined by measuring the change in color of the test paper onto which blood is spotted. Further, as the method of measurement, a light-emitting element such as an LED is used to illuminate the test paper, and the intensity of the reflected light is detected using a light-receiving element such as a light receiving diode.
However, light intensity emitted by light-emitting elements such as an LED can decrease due to generation of heat during light emission. The loss in light intensity of an LED is especially large right after power activation (i.e., turning on the LED). Further, the blood sugar level is calculated by comparing the measurement data prior to spotting blood with the measurement data in the state that chemical reaction stablizes after spotting blood. Accordingly, decrease in the light intensity of an LED during measurement leads to problems in accuracy of measurement data. In order to stabilize light intensity during measurements, it is necessary to carry out light intensity stabilization processing prior to measurements after the device is switched on. The patent reference 1 shows a technique that measurements of reflected light is made after the light intensity emitted from the light source has stabilized.
Patent reference 2 discloses a body fluid constituents measurement device in which the light-emitting element is pulse-driven in bursts at intervals, and the average of reflected light intensities from a plurality of pulse signals are obtained for calculating measurement values such as blood glucose level. The body fluid constituents measurement device of patent reference 2 initiates measurement after a predetermined period of time has passed since switching on the light-emitting element, and averages the light intensities of reflected light during each burst, thereby improving accuracy. Further, the body fluid constituents measurement device of patent reference 2 shortens measurement time by initiating measurement prior to stabilization of the emission condition of the light-emitting element.
Patent reference 3 discloses a body fluid constituents measurement device which is easy to operate and is highly accurate. The body fluid constituents measurement device of patented reference 3 quantifies constituents by obtaining differences between the intensities of reflected light measured when pulsed light is not emitted and when pulsed light is emitted. Further, the body fluid constituents measurement device of patent reference 3 improves accuracy of the measurement result by measuring the light pulsed at a interval which is half cycle of the commercial alternating current or its integral multiples when measuring the difference in intensities of the reflected light.
Patent reference 1: Japanese Patent Laid-open No. H03-73828
Patent reference 2: Japanese Patent Laid-open No. 2002-168862
Patent reference 3: Japanese Patent Laid-open No. H10-318928